1. Field of the Invention
The present invention relates to an apparatus for checking defects involved or residing in a mask and/or reticle used for the manufacture of semi-conductor elements, and, in particular, for detecting foreign particles stuck to the mask and/or reticle. More specifically, the present invention relates to an apparatus for checking a mask and/or reticle covered with a transparent film or pellicle to protect the mask and/or reticle against fine dust floating in the environment, or the transparent film or pellicle itself.
2. Related Background Art
Apparatuses for automatically detecting foreign particles stuck to a mask and/or reticle (referred to generically as "reticle" hereinafter) by scanning the reticle by means of light beams are already known, as disclosed in the U.S. Pat. Nos. 4,468,120 and 4,610,541. Such conventional apparatus checks the existence of the foreign particles, size of the particles and sticking condition of the particles, by linearly scanning a surface of the reticle in x-direction by means of a galvano-mirror (vibration mirror) and the like, while directing a spot of condensed laser beam onto said surface at an incident angle of 10.degree. to 20.degree., by shifting the reticle in y-direction, and then by receiving the light scattered laterally and rearwardly (among the reflected light) by means of a plurality of photoelectric detectors arranged in a space position related specially with respect to the position of illumination of the laser beam.
In such checking apparatuses, it has recently been desirable to enable effective checking of a reticle with a thin film of polymeric material (referred to as "pellicle" hereinafter).
The pellicle is fixedly supported on the reticle by a supporting frame so that the pellicle covers the reticle leaving a space between the pellicle and a surface of the reticle, thereby preventing adhesion or sticking of the foreign particles on the reticle. When a projecting and exposing operation is performed by means of an exposing device making use of such pellicle, even if the foreign particles are stuck or adhered to the surface of the pellicle, an image of the foreign particles is not transferred onto a semi-conductor wafer which receives projected images, since the image of the foreign particles is not focused on the wafer.
However, when the size of the foreign particles stuck to the surface of the pellicle is relatively large, there would be patches on the semi-conductor wafer (uneven exposure). Further, when some of the foreign particles stuck to a lower surface of the pellicle (i.e., a surface facing the reticle) drop from the surface of the pellicle onto the reticle, even if they are not so large as to create uneven exposure, the image of the dropped particles would be transferred to the semi-conductor wafer.
Therefore, even if the pellicle is used, it is necessary to check or inspect the position and size of the foreign particles stuck to the pellicle, and further to judge or discriminate whether the foreign particles are stuck to an upper surface (remote from the reticle) of the pellicle or a lower surface (facing the reticle) of the pellicle.
However, such discrimination could not be obtained by the conventional checking apparatuses.
Further, when the reticle covered with the pellicle was checked by the conventional apparatuses without any additional devices, the inventors of the present invention have found the following disadvantages.
Firstly, since the laser beam or light advances obliquely from the pellicle to the reticle, an incident angle of the laser beam would be changed in accordance with the scanning positions, thereby changing light intensity of the spot of the laser beam reaching the reticle surface, due to the fact that light permeability of the pellicle is seemingly changed by the change of the incident angle of the laser beam with respect to the pellicle.
Secondly, since the photoelectric detectors are arranged in predetermined space position, an angle formed between the pellicle and a light path of the scattered light sent from the foreign particles to the photoelectric detectors is changed in accordance with the sticking position of the foreign particles, and accordingly the scanning position of the light spot, due to the fact that light permeability of the pellicle is seemingly changed by changing an incident angle of the scattered light with respect to the pellicle.
Accordingly, even if the foreign particle is stuck in the same position, there arises a problem that the sensitivity of the detection of the foreign particle between the reticle with the pellicle and the reticle without the pellicle is different.
Further, since the supporting frame for supporting the pellicle is arranged around a pattern area of the reticle, when the pellicle is mounted on the reticle, there arose a defect that a part of the scattered light from the foreign particles stuck to the reticle is interrupted by the supporting frame so as not to reach the photoelectric detectors. In order to correct the defect, a method in which after the reticle is checked, the reticle is rotated at an angle of 90.degree. or 180.degree., and then the reticle is checked again can be adopted; however, this method is time consuming, and thus, is not desirable.
Furthermore, in the conventional apparatus for detecting the foreign particles, a light beam having the same diameter has been used both for checking the reticle surface and for checking the pellicle surface.
In general, the sensitivity of the detection of the foreign particle depends upon the beam diameter of the light beam for scanning the surface to which the foreign particle is stuck; the smaller the beam diameter the higher the sensitivity of the detection of the foreign particle, thus enabling the detection of the smaller foreign particle. On the other hand, the smaller the beam diameter, the longer the time for evenly scanning the whole surface to be checked.
As described above, the foreign particles stuck to the pellicle are not easily projected onto the semi-conductor wafer (in comparison with the foreign particles stuck to the reticle), even if the particles are relatively large. In other words, an allowable size of the foreign particle stuck to the reticle differs from that of the pellicle; the pellicle is permitted to have the larger particles than the reticle.
Accordingly, when the pellicle surface is checked by the above-mentioned conventional detecting apparatus, the foreign particles which are smaller than those that need to be detected can also be detected, with the result that there arose a problem that it took longer for the detection of the foreign particles than normally needed.